Stirling cycle cryocooler with improved magnet ring assembly and gas bearings

ABSTRACT

A piston assembly for use in a motor comprises a cylinder having a bore, an electrically conductive piston reciprocally disposed within the cylinder bore, a gas cavity formed within the piston, and one or more gas bearings associated with the piston, each of the one or more gas bearings including an aperture formed within the piston and an electrically conductive tubular member extending through the aperture, the tubular member having a lumen in communication between the gas cavity and the cylinder bore. The piston assembly may be used in connection with a motor used in a cryocooler that is driven by oscillating magnetic energy fields.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/160,570, filed May 30, 2002, now U.S. Pat. No. 6,694,730. The '570application is incorporated by reference as if set forth fully herein.

FIELD OF THE INVENTION

The field of the invention relates generally to cryocoolers, and moreparticularly to Stirling cycle cryocoolers.

BACKGROUND OF THE INVENTION

Recently, substantial attention has been directed to the field ofsuperconductors and to systems and methods for using such products.Substantial attention also has been to directed to systems and methodsfor providing a cold environment (e.g., 77 K or lower) within whichsuperconductor products such as superconducting filter systems mayfunction.

One device that has been widely used to produce a cold environmentwithin which superconductor devices may function is the Stirling cyclerefrigeration unit or Stirling cycle cryocooler. Such devices typicallycomprise a displacer unit and a compressor unit, wherein the two unitsare in fluid communication and are driven by one or more linear orrotary motors. Conventional displacer units generally have a “cold” endand a “hot” end, the warm end being in fluid communication with thecompressor unit. Displacer units generally include a displacer having aregenerator mounted therein for displacing a fluid, such as helium, fromone end, i.e., the cold end of the displacer unit, to the other end,i.e., the warm end, of the displacer unit. A piston assembly of themotor functions to apply additional pressure to the fluid when the fluidis located substantially within the warm end of the displacer unit, andto relieve pressure from the fluid when the fluid is locatedsubstantially within the cold end of the displacer unit. In typicalcryocoolers, the piston and displacer units oscillate at 60 Hz. In thisfashion, the cold end of the displacer unit may be maintained, forexample, at 77 K, while the warm end of the displacer unit ismaintained, for example, at 15 degrees above ambient temperature.Devices such as superconducting filters are then typically placed inthermal contact with the cold end of the displacer unit via a heatacceptor. Heat is transferred from the device to the heat acceptor. Theheat transferred to the heat acceptor then passes to the helium gascontained in the displacer unit.

A typical motor used in a cryocooler comprises a piston assembly onwhich there is mounted a magnet ring assembly that transforms anoscillating magnetic energy field generated by motor coils toreciprocating mechanical energy that is applied to the piston assembly.For example, FIGS. 1 and 2 illustrate a prior art piston/magnet assembly10, which includes a piston assembly 12 and a magnet ring assembly 14mounted thereon. Referring specifically to FIGS. 3-6, the magnet ringassembly 14 includes eight magnets 16 that are cylindrically arranged toprovide a radial magnetic field. To affix the magnets 16 in place, themagnet ring assembly 14 comprises an upper magnet holder 18, whichincludes an annular recess 20 that captures the tops 22 of the magnets16, and a lower magnet holder 24, which includes an annular recess 26that captures the bottoms 28 of the magnets 16. Preferably, the wallsthat straddle the annular recesses 20 and 26 are as thin as possible(e.g., 0.0050 inch), so that the thickness of the magnets 16 can bemaximized. For purposes of structural integrity, the magnets 16 are heldin place by bonding the tops 22 and bottoms 30 of the magnets 16 withinthe respective annular recesses 20 and 26. The magnet ring assembly 14further comprises eight ring rods 32, which are located between therespective eight magnets 16 and TIG welded through corresponding holes34 within the upper and lower magnet holders 18 and 24 to maintain thestructural integrity of the magnet ring assembly 14.

Referring back to FIGS. 1 and 2, the piston assembly 12 comprises acylinder 36 having a bore 38, a cylindrical piston 40 that axially moveswithin the bore 38 of the cylinder 36, a piston end cap 42 disposedmounted in the end of the piston 40, and a piston bracket 44 disposed onthe opposite end of the piston 40. As best shown in FIGS. 1 and 4, theupper magnet holder 18 of the magnet ring assembly 14 comprises eightradially to circumferentially disposed mounting apertures 46, and thepiston bracket 44 comprises eight corresponding circumferentiallydisposed mounting apertures 48, which are used to firmly bolt the magnetring assembly 14 to the piston assembly 12, as illustrated in FIG. 1. Sothat the top surface of the upper magnet holder 18 is flush with themounting surface of the piston bracket 44, the piston bracket 44 furtherincludes eight radially disposed apertures 50 between the mountingapertures 48 to accommodate the ends of the ring rods 32 (shown best inFIG. 3) protruding through the upper magnet holder 18.

Referring still to FIG. 2, the piston assembly 12 further comprises gasbearings 52 that receive gas, e.g., helium, from a sealed cavity 54within the piston 40. It should be noted that any suitable of gasbearings 52 can be used. In the illustrated embodiment, fourcircumferentially disposed pairs of gas bearings 52 (only two pairsshown) are used. A check valve 56 (best shown in FIG. 1) provides aunidirectional flow of gas from the front of the piston 40, through thesealed cavity 54 and out through the gas bearings 52. Preferably, thegas bearings 52 comprise orifices that are on the order of a one or twomils (e.g., 1.5 mils), so that only a small amount of gas escapes fromthe sealed cavity 54 though the gas bearings 52, thereby preserving thepressure that has built up in the sealed cavity 54 until the next strokeof the piston 40. Typically, only 2-5 percent of gas that is displacedby the piston 40 enters the sealed cavity 54 through the check valve 56.

Because the smallest drill bit currently is around 2.9 mils with amaximum length of about 30 mils, the orifices of the gas bearings 52cannot be drilled. Instead, each of the gas bearings 52 includes anaperture 58 in which there is disposed a gas bearing restrictor in theform of a screw 60 that can be turned to adjust the rate of gas thatflows through the gas bearing 52. That is, the length of the passagecreated by the threaded helix between the screw 60 and the aperture 58can be decreased or increased by carefully rotating the screw 60 in andout of the aperture 58 until the correct flow rates are attained in allgas bearings 52. Alternatively, sapphire/ruby or glass orifices (notshown) with very small diameters can be used as the gas bearingrestrictor to provide a consistent gas flow at the designed rate withoutrequiring adjustment. These orifices, however, can only be made so long,and as will be described in more detail below, have reliabilityproblems. The piston assembly 12 further comprises centering ports 62(shown in FIG. 1), which provide a return gas circuit from regionadjacent the back of the piston 40 to the region adjacent the front ofthe piston 40.

Due to the tight tolerances (typically, about 5 mils) between the magnetring assembly 14 and adjacent laminations (only internal lamination 28shown) that are disposed on both the inside and outside surface of themagnet ring assembly 14, the circularity of the magnet ring assembly 14must be perfect or near-perfect, so that it does not rub against theadjacent laminations. For the same reason, the concentricity between thepiston 40 and the magnet ring assembly 14 must be perfect ornear-perfect. In addition, the magnets 16 must be in a perfect ornear-perfect cylindrical equidistant arrangement, so that the generatedmagnetic field is radially uniform. In this manner, a uniform load willbe provided to the gas bearings 52, thereby maximizing the efficiency ofthe piston assembly 12. Thus, it can be appreciated that great care mustbe taken when assembling the magnet ring assembly 14, resulting in oftentedious and time consuming process that is magnified by the relativelylarge number of parts (eighteen-eight magnets, eight ring rods, twomagnet holders) that make up the magnet ring assembly 14. Notably,magnet segments cannot currently be made as a single fully cylindricalpiece due to magnetic technology limitations. Thus, multiple magnetsmust be painstakingly mounted within the upper and lower magnet holders18 and 24. Also, the measures taken to ensure that the magnet ringassembly 14 and piston 40 are concentric along their lengths, namely,the drilling of the apertures 50 within the piston bracket 44 thataccommodate the protruding ring rods 32, provide additionaltime-consuming steps. Furthermore, because the walls adjacent theannular recesses 20 and 26 of the respective upper and lower magnetholders 18 and 24 are preferably very thin, so that the thickness of themagnets 16 can be maximized, these walls are often inadvertentlyperforated, resulting in the scrapping of the respective magnet holder.

In addition, all eight screws 60 within the apertures 58 of the gasbearings 52 have to be iteratively adjusted and the flow rate measuredthroughout the fabrication process of the cryocooler to ensure that thegas bearings 52 exhibit the designed flow rate at the end of the finalassembly process. Great care must be taken when rotating the screws 60within the apertures 58, so that the heads of the screws 60 are notstripped. Occasionally, however, this will occur, requiring that theexpensive piston assembly 12 be scrapped.

Reliability of the cryocooler is another concern. In the field ofcommercial Radio Frequency (RF) communications, it is desired thatStirling cycle cryocoolers provide maintenance free operation for tensof thousands of hours, and more preferably, at least forty thousandhours. After mere thousands of operational hours, however, cryocoolersthat incorporated piston/magnet assemblies similar to the assembly 10described above were failing. It was discovered that, when the piston 40banged against the cylinder 36, the epoxy joints between the magnets 16and the upper and lower magnet holders 18 and 24 would break and/or themagnet ring assembly 14 would go out of round, causing the magnet ringassembly 14 to rub against the adjacent laminations and/or unequalloading of the gas bearings 52. As a result, the magnet ring assembly 14would deteriorate rapidly. Thus, the high energy transmitted to themagnet ring assembly 14 due to the high frequency application of themotor stresses the importance of the attachment technique between magnetand the magnet holder. It was also discovered that when sapphire/ruby orglass orifices are alternatively used as the gas restrictors, a staticcharge would build up as the gas flows through them at 60 Hz. As aresult, very fine particles would collect within the very smalldiameters (typically about 0.0012 inch in diameter) and eventually plugthem.

Thus, there is a need for an improved magnet ring assembly and gasbearing restrictor that can be used with piston assemblies, such asthose found in cryocoolers.

SUMMARY OF THE INVENTION

The present inventions are directed to magnet ring assemblies andpiston/magnet assemblies, motors, and cryocoolers that utilize suchmagnet ring assemblies. In accordance with the present inventions, amagnet ring assembly comprises a cylindrical magnet holder having aninner surface, and one or more magnets disposed around the inner surfaceof the cylindrical magnet holder. In the preferred embodiment, aplurality of equidistantly spaced magnets is disposed around the innersurface of the cylindrical magnet holder. So that the magnets conform tothe cylindrical magnet holder, each of the plurality of magnets ispreferably arcuate and comprises an outer radius of curvaturesubstantially equal to the inner radius of the cylindrical magnetholder. The magnets can be captured by the magnet holder in a variety ofdirections.

For example, the magnets can be rotationally captured by bonding them tothe inner surface of the magnet holder. The magnets can be radiallycaptured by providing the plurality of magnets with a radially uniformmagnet polarity, such that they mutually magnetically repel each otheragainst the inner surface of the cylindrical magnet holder. Also, eachof the magnets can exhibit an outer arcuate length that is greater thanthe inner arcuate length, such that any one of the magnets is capturedby the edges of the adjacent magnets, and thus cannot be displacedradially inward. The magnets can be axially captured by forming anannular ledge on the inner surface of the magnet holder and disposingone of the axial edges of each magnet on the annular ledge, and swagingthe axial edge of magnet holder around the other axial edge of each ofthe magnets.

By way of non-limiting example, the afore-described magnet ring assemblyprovides various advantages. For example, the magnets can bemechanically captured to sustain high frequency operation of the pistonon which the magnet ring assembly is mounted. Also, assuming that themagnet holder is a unibody structure, the number of parts (not includingthe magnet sectors) can be reduced to one, and no TIG welds arerequired. In addition, alignment of the magnet sectors can be easilyaccomplished, since the magnet sectors self-align to each other as theyare inserted into the magnet holder. Also, since the magnet sectors arenot associated with the outer surface of the magnet holder, the outersurface can be grinded, such that it is concentric with the innersurface thereof.

The present inventions are also directed to gas bearing restrictors, andpiston assemblies, motors, and cryocoolers that utilize such gas bearingrestrictors. In accordance with the present inventions, a pistonassembly comprises a cylinder having a to bore, an electricallyconductive piston reciprocally disposed within the cylinder bore, a gascavity formed within the piston, and one or more gas bearings associatedwith the piston. Each of the gas bearings includes an aperture formedwithin the piston and an electrically conductive tubular memberextending through the aperture. The tubular member includes a lumen incommunication between the gas cavity and the cylinder bore. In thepreferred embodiment, the tubular member is a composite tube composed ofouter and inner tubes that are press-fit into the aperture.

By way of non-limiting example, the afore-described gas bearing providesvarious advantages. For example, because the tubular member iselectrically conductive, static buildup is minimized, thereby minimizingthe chances that the orifice will become plugged. In addition, if thetubular member is a composite tube formed of outer and inner tubularmembers, the wall of the composite tube can be made thick for ease ofplacement into the aperture, while forming an inner lumen that isrelatively small. Thus, no adjustment of the gas bearings are required,and at most, one flow measurement needs to be performed, since the sizeof the lumen will not change during the manufacturing process.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is a top view of a prior art piston/magnet assembly;

FIG. 2 is a cross-sectional view of the piston/magnet assembly of FIG.1, taken along the line 2—2;

FIG. 3 is a perspective view of a magnet ring assembly used in thepiston/magnet assembly of FIG. 1;

FIG. 4 is a top view of the magnet ring assembly of FIG. 3;

FIG. 5 is a cross-sectional view of the magnet ring assembly of FIG. 4,taken along the line 4—4;

FIG. 6 is a cross-sectional view of the magnet ring assembly of FIG. 4,taken along the line 5—5;

FIG. 7 is a cross-sectional view of a cryocooler constructed inaccordance with one preferred embodiment of the present inventions;

FIG. 8 is a cross-sectional view of a novel piston/magnet assembly usedin the cryocooler of FIG. 7;

FIG. 9 is another cross-sectional view of the piston/magnet assemblyused in the cryocooler of FIG. 7;

FIG. 10 is still another cross-sectional view of the piston/magnetassembly used in the cryocooler of FIG. 7;

FIG. 11 is a close-up view of a novel gas bearing used in thepiston/magnet assembly of FIG. 8;

FIG. 12 is a plan view of a composite tube used in the gas bearing ofFIG. 11;

FIG. 13 is a perspective view of a novel magnet ring assembly used inthe piston/magnet assembly of FIG. 8;

FIG. 14 is a cross-sectional view of the magnet ring assembly of FIG.13, taken along the line 13—13; and

FIG. 15 is a cross-sectional view of the magnet ring assembly of FIG.13, taken along the line 14—14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 7 illustrates a Stirling cycle cryocooler 100 constructed inaccordance with a preferred embodiment of the present inventions. Asshown, the Stirling cycle cryocooler 100 includes a displacer unit 102that is associated with a cold region P_(COLD) and a warm region P_(HOT)at its opposite ends, a compressor unit 104, which is in fluidcommunication with the displacer unit 102, and a heat exchanger unit 106between the displacer unit 102 and the compressor unit 104. Thecryocooler 100 also includes a passive counterbalancer 107, whichcancels the movement of the moving internal components of the compressorunit 104, thereby minimizing vibration of the cryocooler 100.

The compressor unit 104 comprises a pressure housing assembly 108, motor110, spring assembly 112, and a compression chamber 114 that iscoincident with the warm region P_(HOT). The housing assembly 118comprises a front bracket 120, a rear bracket 122, and a cylindricalhousing section 124 mounted therebetween. The spring assembly 112comprises a spring bracket 126, and a pair of axially spaced flexuresprings 128 and 130 mounted thereon, which as will be described infurther detail below, provide the necessary displacement phase betweenthe compression and displacement functions of the cryocooler 100. Themotor 110 comprises a piston/magnet assembly 132 and a magneticinduction assembly 134, which magnetically communicate with each otherto provide the reciprocating action required to compress the fluid,e.g., gaseous helium, within the compression chamber 114. Thepiston/magnet assembly 132 comprises a piston assembly 136 and anassociated magnet ring assembly 138.

Referring further to FIGS. 8-10, the piston assembly 136 includes acylinder 140 that is mounted to the front bracket 120 of the housingassembly 118 (shown in FIG. 7), a piston 142 slideably disposed within abore 146 of the cylinder 140, and a piston mounting bracket 148 formechanically coupling the piston 142 to the piston flexure spring 128 ofthe spring assembly 112, and for mechanically coupling the magnet ringassembly 138 to the piston 142. The piston 142, along with the magnetring assembly 138, is thus adapted for reciprocating motion within thecylinder 140. The piston 142 comprises a bore 150 in which there isdisposed a displacer lining 152, which as will be described in furtherdetail, is associated with the displacer unit 102 and facilitates thefluid displacement function of the cryocooler.

Referring specifically to FIG. 8, the piston assembly 136 furthercomprises a plurality of gas bearings 154 (in the illustratedembodiment, four pairs of gas bearings) that are circumferentiallydisposed about and circumferentially formed around the piston 142 in anequidistant manner, a substantially sealed cavity 156 formed within thepiston 142 for providing gas, e.g., helium, to the gas bearings 154, anda check valve 158 that provides a unidirectional fluid communicationconduit from the warm region P_(HOT) (i.e., the compression to chamber114) to the sealed cavity 156 when the pressure of the gas within thatregion exceeds the pressure within the cavity 156 (i.e., exceeds thepiston reservoir pressure). Thus, it can be appreciated that when thepiston 142 moves towards the compression chamber 114, the gas from thecompression chamber 114 is forced through the check valve 158, into thesealed cavity 156, and out through the gas bearings 154.

With specific reference to FIG. 11, the detailed structure of one of thegas bearings 154 will now be described. The gas bearing 154 comprises abearing space 160 formed within the external surface 162 of the piston142, an aperture 164 transversely extending from the bearing space 160through the wall 164 of the piston 142 and into the sealed cavity 156,and a composite tube 166 that extends through the aperture 164. Thecomposite tube 166 comprises a lumen 168 that is in communicationbetween the bearing space 160 and the sealed cavity 156 to provide aflow of gas from the sealed cavity 156 into the cylinder 140.

Preferably, the aperture 164 is formed by transversely drilling a holethrough wall 164 of the piston 142. In the illustrated embodiment, thehole has a diameter of approximately 0.020 inch and a length of 0.100inch. The outer diameter and length of the composite tube 166 isapproximately 0.020 inch and 0.100 inch, respectively, and the diameterof the lumen 168 is approximately 0.0012 inch. Thus, the relativelythick wall of the composite tube 166, which in the illustratedembodiment is approximately 0.0094 inch thick, allows the composite tube166 to be easily press-fit into the aperture 164. Significantly, thecomposite tube 166 is composed of an electrically conductive material,such as, e.g., stainless steel. As a result, the composite tube 166 iselectrically grounded through the electrically conductive piston 142,and thus, a “static charge” will not build up, thereby preventing or atleast minimizing the collection of dust particles within the lumen 168.Also, because the diameter of the lumen 168 does not change during themanufacturing process of the cryocooler 100, and is consistentthroughout any given run of a tubing, flow measurements for each gasbearing 154 need not be performed, or at the most performed only once,thus reducing cost.

Referring to FIG. 12, the composite tube 166 can be advantageouslycomposed of an outer tubular member 170 and an inner tubular member 172to provide the proper wall thickness of the composite tube 166, whileallowing for a very small diameter lumen 168. Specifically, tomanufacture the composite tube 166 with exemplary inner and outerdiameters of 0.0012 inch and 0.020 inch, long lengths of stainless steeltubing, similar to “hypodermic needle tubing,” can be fabricated with aninner diameter of 0.0015 inch and an outer diameter of 0.0070 inch toform the inner tubular member 172. Long lengths of stainless steeltubing can be fabricated with an inner diameter of 0.0075 inch and anouter diameter of 0.0020 inch to form the outer tubular member 170. Theouter tubular member 170 is then swaged over the inner tubular member172 to form a long length of the thick wall composite tube 166, whichwill have an outer diameter of 0.020 inch, an inner diameter ofapproximately 0.0012 inch (reduced from 0.0015 inch due to the swaging),and a wall thickness of 0.0094 inch. The length of the composite tube166 is then cut into 0.100 inch lengths, the ends of which can bechemically etched to provide multiple burr-free composite tubes 166.Alternatively, the length of the composite tube 166 can be cut using“wire electric discharge machining” to provide for a multiplicity ofburr-free composite tubes 166. The lengths of the composite tubes 166are selected to provide the exact flow rate through to the lumen 168 ofthe composite tube 166. The composite tubes 166 are then press-fit intothe drilled apertures 164 within the piston 142. A suitable manufacturerfor fabricating the composite tube 166 is Phillips & Johnston, Inc.,located in Glen Ellyn, Ill.

Referring specifically to FIGS. 9 and 10, the piston assembly 136further comprises a pair of front centering port assemblies 174 (FIG. 9)and a pair of rear centering port assemblies 176 (FIG. 10) to provide apressure release conduit between the space 178 at the rear end of thecompressor unit 104 and the compression chamber 114. Specifically, eachfront centering port assembly 174 includes double transverse ports 180that communicate with the cylinder 140, and a lumen 182 that axiallyextends within the front 179 of the piston 142 and providescommunication between the double ports 180 and the compression chamber114. Each rear centering port assembly 176 includes double transverseports 184 that communicate with the cylinder 140, and a lumen 186 thataxially extends within the rear 181 of the piston 142 and providescommunication between the double ports 182 and the rear space 178 in thecompressor unit 104. The double ports 180 and 184 communicate with eachother through an annular indentation (not shown) formed on the innersurface of the cylinder 140, so that the rear space 178 momentarilycommunicates with the compression chamber 114 as the piston reciprocallymoves within the cylinder 140, thereby equalizing the pressure betweenthe rear space 178 and the compression chamber 114. Notably, the axialdisplacement between each of the double ports 180 or 184 provide aself-compensating air flow over an operating range of the piston 142.That is, only one port from each of the double ports 180 and 184 provideair flow during low piston 142 strokes, while both ports from each ofthe double ports 180 and 184 provide air flow during high piston 142strokes. In this manner, the piston 142 is not axially biased towardsthe compression chamber 114 by pressure that may otherwise build up inthe rear space 178 as gas flows from the gas bearings 154 into the rearspace 178.

With specific reference to FIGS. 13-15, the magnet ring assembly 138will now be described. The magnet ring assembly 138 comprises a unibodycylindrical magnet holder 188 and a plurality of arcuate magnet sectors190 mounted within the magnet holder 188. In the illustrated embodiment,eight magnet sectors 190 are used, but it should be understood, that anynumber of magnet sectors 190 can be used to provide the proper magneticinteraction with the magnetic induction assembly 134. The eight magnetsectors 190 are circumferentially disposed about the inner surface 192of the magnet holder 188 in a circular equidistant pattern. Each of themagnet sectors 190 exhibits an outer radius of curvature r₁, and has anouter surface 191 within an outer arcuate length l_(o) and an innersurface 192 with an arcuate length l₁. So that the outer surfaces 191 ofthe magnet sectors 190 are flush within the inner surface 192 of themagnet holder 188, the outer radius of curvature r₁, for each of themagnet sectors 190 is equal to the inner radius r2 of the magnet holder188.

The magnet holder 188 is composed of a high-resistivity material (≧70microhm cm), such as, e.g., stainless steel or any non-magneticmaterial. In this manner, magnetic losses through the magnet holder 188are minimized. To further reduce the magnetic losses, the wall thicknessof the magnet holder 188 surrounding the magnet sectors 190 is reduced,e.g., to less than 0.012 inch, by machining the outer surface 194 of themagnet holder 188. The inner surface 192 of the magnet holder 188 ismachined to establish the true position to outer diameter needed foralignment of the piston 142 with the cylinder 140.

The eight magnet sectors 190 are affixed in place in three directions:the axial direction (Z-direction), rotational direction (θ-direction),and the radial direction (r direction). In the axial direction, themagnet sectors 190 are axially captured from both ends to eliminate anychance of escape due to the alternating axial motion of the magnet ringassembly 138. Specifically, each of the magnet sectors 190 comprisesopposing axial edges 196 and 198, one of which is axially affixed in thefirst direction by an annular ledge 200 formed around the inner surface192 of the magnet holder 188, and the other of which is axially affixedin the second direction by swaging the axial edge 202 of magnet holder188 inward. In addition to capturing the magnet sectors 190, the swagedaxial edge 202 provides structural integrity to the magnet holder 188,so that the magnet ring assembly 138 maintains circularity. In therotational direction, the magnet sectors 190 are bonded to the innersurface 192 of the magnet holder 188 using a suitable bonding material,such as, e.g., epoxy, which exhibits good shear strength at hightemperatures. In the radial direction, the arrangement of the magnetsectors 190 have a uniform radial polarity. In the illustratedembodiment, the polarity of the magnet sectors 190 is oriented with theNorth Pole pointing outward and the South Pole pointing inward. Thus,the uniform radial polarity repels each magnet sector 190 from the othermagnet sectors 190 towards the inner surface 192 of the magnet holder188. In this manner, the outwardly radial magnetic force facilitates thesecuring of the magnet sectors 190 to the magnet holder 188. Inaddition, because each of the magnet sectors 190 has an outer arcuatelength l_(o) that is greater than an inner arcuate length l_(i)., aninterference fit is provided between adjacent magnet sectors 190,thereby preventing the magnet sectors 190 from being radially displacedfrom the adjacent magnet sectors 190.

Thus, it can be appreciated that the magnet sectors 190 are mechanicallycaptured to sustain high frequency operation of the piston 142. Otheradvantages are provided by the magnet ring assembly 138. For example,compared to the prior art magnet ring assembly 138 illustrated in FIGS.3-6, the number of parts (not including the magnet sectors 190) isreduced from ten to one, and 16 TIG welds are eliminated, thereby alsoeliminating the need to drill apertures within the piston mountingbracket 148 in order to accommodate ring rods. In addition, alignment ofthe magnet sectors 190 is easily accomplished, since the magnet sectors190 self-align to each other as they are inserted into the magnet holder188. Also, since the magnet sectors 190 are not associated with theouter surface 194 of the magnet holder 188, the outer surface 194 can begrinded, such that it is concentric with the inner surface 192 thereof.

Referring back to FIG. 7, the magnetic induction assembly 134 comprisesinternal laminations 208 mounted to the outside of the cylinder 140,external laminations 210 that are mounted between the front and rearmotor brackets 120 and 122 in close outward proximity to the magnet ringassembly 138 to form a gap (not shown), and a motor coil 212 that lieswithin the recesses formed within the external laminations 210 andsurrounds the magnet ring assembly 138. The internal and externallaminations 208 and 210 are preferably composed of a ferrous material.Thus, it will be appreciated that as the electrical polarity of the coil212 is alternately switched back and forth, the resulting magnetic forcethat is applied to the magnet ring assembly 138 across the gap changes.As a result, the magnet ring assembly 138 reciprocally moves within thegap, and the piston 142 accordingly reciprocally moves within thecylinder 140.

The displacer unit 102 functions in a conventional manner and includes adisplacer housing 214, a displacer cylinder assembly 216, a displacerrod 218, and a heat acceptor 220. The displacer cylinder assembly 216comprises a displacer body 222 that is slideably mounted within thedisplacer housing 214, and a regenerator 224 mounted within thedisplacer body 222. The displacer body 222 rests against a displacerliner 226 affixed to an inner wall 228 of the displacer housing 214. Thedisplacer rod 218 is slideably disposed within the displacer liner 152mounted within the piston bore 150, and is coupled at one end 230 to abase section 231 of the displacer body 222 and coupled at the other end232 to the displacer flexure spring 130. Thus, under appropriateconditions, it is possible for the displacer body 222 to oscillatewithin the displacer housing 214.

The heat acceptor 220 includes a radial component 234 and an annularcomponent 236. The radial component 234 is generally perpendicular tothe long axis of the displacer unit 102. The annular component 236extends from the radial component 234 and extends axially beyond theedge of the displacer cylinder assembly 216, abutting against a distalend 238 of the displacer liner 226. The heat acceptor 220 is preferablybrazed to the displacer housing 214 to provide a hermetically sealedenvironment. The heat acceptor 220 is preferably made from high puritycopper or oxygen-free-high-conductivity (OFHC) copper. The displacercylinder assembly 216 includes a plurality of radial holes 240. Theradial holes 240 permits additional flow of helium within the coldregion P_(COLD), impinging directly on the heat acceptor 220. The radialholes 240 assist in decreasing the convective resistance between theheat acceptor and the helium gas within the cryocooler 100. Thestructure and function of the heat acceptor 220 is discussed in furtherdetail in U.S. Pat. No. 6,327,862 entitled “Stirling Cycle CryocoolerWith Optimized Cold End Design,” which is hereby expressly incorporatedherein by reference.

The heat exchanger unit 106, which is located between the displacer unit102 and the compressor unit 104, includes a heat exchanger block 242 anda flow diverter 244. The heat exchanger block 242 is mounted to thefront bracket 120 of the compressor unit 104, and includes a pluralityof internal heat exchanger fins 244 and a plurality of external heatrejector fins 246. Thus, the heat exchanger unit 106 is designed tofacilitate heat dissipation from a gas, such as helium, that iscompressed in the warm region P_(HOT) located at the juncture betweenthe displacer unit 102 and the compressor unit 104 (the region P_(HOT)also is referred to herein as the compression chamber 114). Preferably,the heat exchanger block 242, internal heat exchanger fins 244 andexternal heat rejector fins 246 are made from a thermally conductivemetal such as high purity copper.

During operation, the piston 142 and displacer cylinder assembly 216preferably oscillate at a resonant frequency of approximately 60 Hz andin such a manner that the oscillation of the displacer cylinder assembly216 is approximately 90° out of phase with the oscillation of the piston142. Stated somewhat differently, it is preferred that the motion of thedisplacer cylinder assembly 216 will “lead” the motion of the piston 142by approximately 90°.

Those skilled in the art will appreciate that, when the displacercylinder assembly 216 moves to the cold region P_(COLD), most of thefluid, e.g. helium, within the system moves around the flow diverter 244and through the internal heat exchanger fins 244 into the warm regionP_(HOT). Due to the phase difference between the motion of the displacercylinder assembly 216 and the piston 142, the piston 142 should be atmid-stroke and moving in a direction toward the heat acceptor 220 whenthe end of the displacer cylinder assembly 216 is located near the heatacceptor 220. This causes the helium in the warm region P_(HOT), i.e.,the compression chamber 114, to be compressed, thus raising thetemperature of the helium. The heat of compression is transferred fromthe compressed helium to the internal heat exchanger fins 244 and fromthere to the heat exchanger block 242 and external heat rejector fins246. From the heat rejector fins 246, the heat is transferred to ambientair. As the displacer cylinder assembly 216 moves to the warm regionP_(HOT), the helium is displaced to the cold region P_(COLD). As thehelium passes through the displacer body 222, it deposits heat withinthe regenerator 224, and exits into the cold region P_(COLD) atapproximately 77 K. At this time, the compressor piston 142 preferablyis at mid-stroke and moving in the direction of the spring assembly 112.This causes the helium in the cold region P_(COLD) to expand furtherreducing the temperature of the helium and allowing the helium to absorbheat. In this fashion, the cold region P_(COLD) functions as arefrigeration unit and may act as a “cold” source.

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

1. A piston assembly for use in a motor, comprising: a cylinder having abore; an electrically conductive piston reciprocally disposed within thecylinder bore; a gas cavity formed within the piston; and one or moregas bearings associated with the piston, each of the one or more gasbearings including an aperture formed within the piston and anelectrically conductive tubular member extending through the aperture,the tubular member having a lumen in communication between the gascavity and the cylinder bore.
 2. The piston assembly of claim 1, whereinthe lumen has an inner diameter equal to or less than 0.0020 inch. 3.The piston assembly of claim 1, wherein the lumen has an inner diameterequal to or less than 0.0015 inch.
 4. The piston assembly of claim 1,wherein the tubular member has a diameter equal to or less than 0.0200inch.
 5. The piston assembly of claim 1, wherein the tubular member hasa length between 0.100 inch and 0.200 inch.
 6. The piston assembly ofclaim 1, wherein the tubular member is press fit into the aperture. 7.The piston assembly of claim 1, wherein the tubular member is acomposite tube composed of an inner tubular member and an outer tubularmember.
 8. The piston assembly of claim 1, wherein the one or more gasbearings comprises a plurality of gas bearings.
 9. The piston assemblyof claim 8, wherein the plurality of gas bearings are equidistantlydisposed around a circumference of the piston.
 10. The piston assemblyof claim 1, further comprising a unidirectional check valve incommunication between the exterior of the piston and the gas cavity. 11.The piston assembly of claim 1, wherein each of the one or more gasbearings further comprises a bearing space formed within the piston,wherein the lumen is in communication with the cylinder bore via thebearing space.
 12. A piston assembly for use in a motor, comprising: acylinder having a bore; an electrically conductive piston reciprocallydisposed within the cylinder bore; a gas cavity formed within thepiston; and one or more gas bearings associated with the piston, each ofthe one or more gas bearings including an aperture formed within thepiston and an electrically conductive composite tube extending throughthe aperture, the composite tube comprising an outer tubular member andan inner tubular member, the inner tubular member having a lumen incommunication between the gas cavity and the cylinder bore.
 13. Thepiston assembly of claim 12, wherein the lumen has an inner diameterequal to or less than 0.0020 inch.
 14. The piston assembly of claim 12,wherein the lumen has an inner diameter equal to or less than 0.0015inch.
 15. The piston assembly of claim 12, wherein the inner tubularmember has an outer diameter less than 0.010 inch.
 16. The pistonassembly of claim 12, wherein the inner tubular member has an outerdiameter equal to or less than 0.007 inch.
 17. The piston assembly ofclaim 12, wherein the composite tube has a diameter equal to or lessthan 0.0200 inch.
 18. The piston assembly of claim 12, wherein thecomposite tube has a length between 0.100 inch and 0.200 inch.
 19. Thepiston assembly of claim 12, wherein the composite tube is press fitinto the aperture.
 20. The piston assembly of claim 12, wherein theouter tubular member is swaged onto the inner tubular member.
 21. Thepiston assembly of claim 12, wherein the one or more gas bearingscomprises a plurality of gas bearings.
 22. The piston assembly of claim21, wherein the plurality of gas bearings are equidistantly disposedaround a circumference of the piston.
 23. The piston assembly of claim12, further comprising a unidirectional check valve in communicationbetween the exterior of the piston and the gas cavity.
 24. The pistonassembly of claim 12, wherein each of the one or more gas bearingsfurther comprises a bearing space formed within the piston, wherein thelumen is in communication with the cylinder bore via the bearing space.25. A cryocooler, comprising: a compressor unit, including a pistonassembly including a cylinder having a bore, an electrically conductivepiston reciprocally disposed within the cylinder bore, a gas cavityformed within the piston, and one or more gas bearings associated withthe piston, each of the one or more gas bearings including an apertureformed within the piston and an electrically conductive composite tubeextending through the aperture, the composite tube comprising an outertubular member and an inner tubular member, the inner tubular memberhaving a lumen in communication between the gas cavity and the cylinderbore. a magnet ring assembly including a cylindrical magnet holderhaving an inner surface, and a plurality of magnets disposed around theinner surface of the cylindrical magnet holder, the magnet ring assemblybeing mounted to the piston bracket; and one or more electrical coilsthat surround an outer surface of the cylindrical magnet holder of themagnet ring assembly; a displacer unit in fluid communication with thecompressor unit; and a heat exchange unit between the compressor unitand displacer unit.
 26. The cryocooler of claim 25, wherein the lumenhas an inner diameter equal to or less than 0.0020 inch.
 27. Thecryocooler of claim 25, wherein the lumen has an inner diameter equal toor less than 0.0015 inch.
 28. The cryocooler of claim 25, wherein theinner tubular member has an outer diameter less than 0.010 inch.
 29. Thecryocooler of claim 25, wherein the inner tubular member has an outerdiameter equal to or less than 0.007 inch.
 30. The cryocooler of claim25, wherein the composite tube has a diameter equal to or less than0.0200 inch.
 31. The cryocooler of claim 25, wherein the composite tubehas a length between 0.100 inch and 0.200 inch.
 32. The cryocooler ofclaim 25, wherein the composite tube is press fit into the aperture. 33.The cryocooler of claim 25, wherein the outer tubular member is swagedonto the inner tubular member.
 34. The cryocooler of claim 25, whereinthe one or more gas bearings comprises a plurality of gas bearings. 35.The cryocooler of claim 34, wherein the plurality of gas bearings areequidistantly disposed around a circumference of the piston.
 36. Thecryocooler of claim 25, further comprising a unidirectional check valvein communication between the exterior of the piston and the gas cavity.37. The cryocooler of claim 25, wherein each of the one or more gasbearings further comprises a bearing space formed within the piston,wherein the lumen is in communication with the cylinder bore via thebearing space.
 38. A motor, comprising: a piston assembly including acylinder having a bore, an electrically conductive piston that isreciprocally disposed within the cylinder bore, and a piston bracketdisposed on the end of the piston; a gas cavity formed within thepiston; one or more gas bearings associated with the piston, each of theone or more gas bearings including an aperture formed within the pistonand an electrically conductive tubular member extending through theaperture, the tubular member having a lumen in communication between thegas cavity and the cylinder bore; a magnet ring assembly including acylindrical magnet holder having an inner surface and an annular ledgeformed around the inner surface, and a plurality of magnets disposedaround the inner surface of the cylindrical magnet holder, each of theplurality of magnets comprising opposing axial edges, one of the axialedges being disposed on the annular ledge, the magnet ring assemblybeing mounted to the piston bracket; and a magnetic induction assemblyoperably coupled to the magnet ring assembly.
 39. The motor of claim 38,wherein the magnets are equidistantly spaced from each other.
 40. Themotor of claim 38, wherein each of the plurality of magnets is arcuate.41. The motor of claim 40, wherein the cylindrical magnet holder has aninner radius, and each of the plurality of magnets comprises an outerradius of curvature substantially equal to the inner radius of thecylindrical magnet holder.
 42. The motor of claim 40, wherein each ofthe plurality of magnets exhibits an outer arcuate length and an innerarcuate length, the inner arcuate length being less than the outerarcuate length.
 43. The motor of claim 38, wherein the plurality ofmagnets has a radially uniform magnetic polarity.
 44. The motor of claim38, wherein the plurality of magnets is bonded to the inner surface ofthe cylindrical magnet holder.
 45. The motor of claim 38, wherein thecylindrical magnet holder comprises a swaged axial edge opposite theannular ledge, and the other of the axial edges of each of the pluralityof magnets is captured by the swaged axial edge of the cylindricalmagnet holder.
 46. The motor of claim 38, wherein the cylindrical magnetholder is composed of a non-magnetic material.
 47. The motor of claim38, wherein the cylindrical magnet holder is a unibody structure. 48.The motor of claim 38, wherein the magnetic induction assemblycomprises: one or more coils surrounding the piston assembly; one ormore internal laminations adjacent inner surfaces of the plurality ofmagnets; and one or more external laminations surrounding the one ormore coils and being adjacent to the outer surface of the cylindricalmagnet holder of the magnet ring assembly.